1. Field of the Invention
The present invention is directed to a method for determining distortions in an image, as well as to a calibration object suitable for use in such a distortion-determining method.
2. Description of the Prior Art
In all types of imaging methods such as, for example, imaging methods that generate images of regions of a subject using light optics, X-rays, electron beams, magnetic resonance, etc., the imaging accuracy of the acquired image compared to the examined subject is a decisive quality criterion. As a rule, the imaging accuracy is not ideal in all imaging systems, but distortions in the imaging of the subject occur dependent on the displayed image region and the quality of the imaging system under consideration. In the field of magnetic resonance tomography (MRT), for example, the causes for occurring distortions are established by the location-dependent deviations of the basic magnetic field from an ideal value B0 and the likewise location-dependent non-linearities of the gradient system. Typically the distortions of the image that are observed become greater toward the edge of the image region that is presented. The size of the deformation or distortion usually can be described by a dependency on the distance r from the center of the image that is not linear, but of a higher order. The distortion becomes disproportionately greater toward the edge. In, MR the distortions can be described with spherical surface functions of a higher orderwith terms of the 3rd, 5th and 7th order (i.e. proportional to r3, r5 and r7) being the most relevant for modeling in the consideration of non-linearities of the gradient system. The inhomogeneities of a magnet with perfect shimming (i.e. optimally adjusted) are established by terms of the order 8 and above.
The characterization of a particular imaging method under consideration usually covers the description of the realized imaging faithfulness in the form of a quantitative statement of the distortions that occur. As a rule, the deviations of the picture elements from ideal value are thereby recited dependent on the observed image region in absolute or relative values. The necessity of a characterization is particularly important when the imaging method is used, for example, for measuring objects (for example, on the earth's surface) for designing objects (for example, lithography) or in medical diagnostics (for example, computed tomography, x-ray diagnostics, magnetic resonance tomography). In many instances, the determination of the imaging accuracy is in fact possible with good precision; however, the underlying measuring method is complex and often requires specifically fabricated test objects that are not handy and are expensive. In these cases, a check is then usually only possible in a specific environment but not for an arbitrary user of the imaging method within the framework of, for example, a quality or constancy check.
For example, three of the methods currently employed for determining the imaging faithfulness are set forth in brief below.
In the “direct” measurement of the distortion, a known test object having optimally simple geometrical structures is imaged and the distortion of the picture elements observed is directly measured. To this end, the determination of the coordinates of the picture elements and the exact reconstruction of the “ideal” coordinates of the object points that would derive for the case of a true-to-image presentation are needed. However, a pre-requisite for this is that not only the dimensions of the underlying test object that must be exactly known, but also the positioning and alignment of the test object in the imaging volume. This is difficult particularly when the imaging volume has no fixed reference points established a priori and is subject to relatively high imprecision. As a rule, this is the case in all three-dimensional imaging methods.
In the imaging of a uniform grid structure, a two-dimensional or three-dimensional test object having a regular, permanently prescribed arrangement of test points in the form of a grid is employed. This allows the determination of the imaging accuracy over a greater image region. The definition of reference points for the construction of the distortion-free picture elements in Cartesian coordinates can implicitly ensue by using individual object points. However, the problem aligning the object in the direction of the image axes also remains here, since even a slight tilting of the object directly diminishes the precision in the determination of the distortion. A significant disadvantage of this method is also that the preparation of correspondingly exact test objects having many test points, for example a cube having an edge length of 30 cm and a three-dimensional grid space 20 mm is extremely complicated and expensive. When such a phantom is intended to cover a large region of the imaging volume, then a heavy weight of the phantom results, and thus the phantom is difficult manipulation for a user to manipulate.
Finally, various organizations such as, for example, NEMA or the American College of Radiology (ACR) have established measurement rules with which the image quality in magnetic resonance tomography is to be defined. Since, in particular, the ACR in the USA accredits the clinical users of MR in an extensive program, the measuring methods employed have created a type of quasi standard. A simple method upon utilization of a circular or spherical phantom is employed here for evaluating the imaging faithfulness. The determination of the distortion on the circumference of the image circle ensues by repeated distance measurement along the diameter from one point to the point to the point lying opposite. The distortion in the radial direction then is derived by comparison to the known diameter of the test object. The advantage of this method is that a spherical phantom is usually already supplied by the manufacture for each MR system, and the measurement and evaluation are simple to implement. A specific phantom, which represents an additional expense, is employed for the measurements in the case of the cost-incurring ACR accreditation; however, only the distance measurement of the outside edges is likewise employed for the evaluation. A disadvantage in the use of the standard phantoms (for example, 170 mm and 240 mm diameter) and the use of the ACR phantom is that these are significantly smaller then the possible imaging region, which typically has a 500 mm field of view (FOV). The determination of the imaging faithfulness is thus also possible only in a very limited image region. The use of a larger phantoms is fundamentally possible but these are expensive due to their manufacture and are also very heavy above a size of 300 mm diameter and are thus difficult to handle. A further disadvantage of this method is that the distortion, caused by the measurement can be measured only along the radial direction given simultaneous observation of two distorted picture elements. Only the average value of the distortions of the two picture elements and not that of a specific point thus can be measured. Moreover, only the radial component of the distortion of the picture elements is identified, but not the complete shift of the points with magnitude and direction, i.e. vectorially.